Method for producing electrolytically coated cold rolled strip, preferably for use in the production of battery sheaths, and battery sheath produced according to this method

ABSTRACT

The invention relates to a method for producing an electrolytically coated cold rolled strip, preferably for use in the production of battery sheaths. The cold rolled strip is provided with a cobalt or a cobalt alloy layer by an electrolytic method. The aim of the invention is to provide a battery sheath with low values for the electric contact resistance between the cathode substance of the battery and the inner surface of the battery sheath. To this end, organic substances m added to the electrolyte during coating that produce decomposition products, said decomposition produces and/or reaction products of said decomposition products with other components of the electrolytic bath being deposited on the strip material as a brittle layer along with the cobalt or the cobalt alloy.

This invention relates first to a method for producing electrolyticallycoated cold band, preferably used for the manufacturing of batteryshells, during which the cold band is electrolytically coated with alayer of cobalt or cobalt alloy.

BACKGROUND OF THE ART

In general, battery shells are manufactured of a cold bandelectrolytically coated with multiple layers. Document EP 0 629 009 B1describes a cold band as well as a method for producing cold bandelectrolytically plated with nickel that is characterized by a favorablebehavior during a subsequent drawing and ironing process. In order toimprove the effective contact surface between the inner side of thebattery shell and the cathode material, a hard coating of a nickel layeris deposited on the side of the cold band that will later form the innerside of the battery shell. During the drawing and ironing process,cracks form in this layer resulting in an enlargement of the contactsurface.

A similar material is described in document EP 0 809 307 A2. The side ofthe band material that will later form the inner side of the batteryshell is coated with a hard material layer, whereas the other side—whichwill later form the outer side of the battery shell—is coated with arelatively soft metal layer. To achieve a hard metal coating, thisdocument proposes electroplating process with an alloy on the nickelbasis. Document EP 0 809 307 A2 indicates various examples of thehardness of thus produced alloys. It also mentions the possibility toadd organic ingredients to the galvanic bath; however, no indicationregarding the hardness properties of layers produced with these baths ismade. During the subsequent forming of the metal sheet into batteryshells, brittle fractures are supposed to form in the electrolyticallydeposited hard alloy coating which results in an enlarged surface and,therefore, reduced electrical contact resistance between the cathodesubstance of the battery and the inner surface of the battery shell.

In general, the use of organic ingredients in galvanic bath has beenknown for a long time as documented, e.g., by U.S. Pat. No. 2,026,718from 1936. This document describes the addition of aromatic sulfonicacids to the galvanic bath for the purpose of achieving a glossy layer.

The use of organic ingredients in galvanic nickel, cobalt, andnickel-cobalt baths for producing an improved cold band for themanufacturing of battery shells has also been known. So, e.g., documentpatent DE 19 53 707 A1 describes a procedure, during which layers ofnickel, cobalt or of their alloys are deposited after an unsaturated,organic substance such as butyne diol has been added to the electrolyte.This document proposes deposition with inert anodes in a halogen-freebath at a current density of 83.8 A/dm², where the process is controlledin such a manner as to avoid brittle deposits.

Finally, it is known from the state of art how to deposit cobalt from agalvanic bath with the aid of organic substances in order to form, e.g.,ferromagnetic layers for data carriers (see, e.g., U.S. Pat. No.4,756,816).

SUMMARY OF THE INVENTION

The underlying task of this invention is to develop a procedure forelectrolytic production of cold band that would allow—during themanufacturing of battery shells by drawing and ironing—to achieve lowvalues of the electrical contact resistance between the cathodesubstance of the battery and the inner side of the cupular batteryshell. Furthermore, the invention is to propose the manufacturing of abattery shell according to such procedure and by subsequent formingoperation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To resolve this task, the invention proposes, for the procedure of theinitially mentioned nature, that organic substances that createdecomposition products be added to the electrolyte during the coatingprocess and that these decomposition products and/or reaction productsof these decomposition products with other ingredients in the galvanicbath are deposited on the band material together with cobalt or thecobalt alloy.

Thus produced cold band is characterized in that its side coated with anelectrolytic layer, if subjected to strong forming forces as they occurduring drawing and ironing of the material to produce, e.g., a batteryshell, shows an especially low electrical contact resistance. The causeof this phenomenon is that the brittle layer of cobalt or a cobalt alloycracks and forms individual plates separated by fissures. Due to thesefissures, the electrical contact resistance is diminished, which is whythe band produced in a procedure according to this invention isespecially suitable as the basis material for the manufacturing ofcupular battery shells by deep-drawing or drawing and ironing, andespecially for batteries with alkaline electrolytes. Compared to thecurrent state of art, the inner side of the thus manufactured batteryshell demonstrates even lower values of the electrical contactresistance between the cathode substance of the battery and the innerside of the battery shell.

In order to achieve the desired brittleness and to improve theelectrical conductivity of the electrolytically produced layer, theprocess conditions should preferably be selected in such a manner thatthe resulting coating contain a cobalt share of more than 35 weightpercent. Furthermore, it is advantageous if the current density of theelectrolyte bath lies within a relatively narrow range of no more than10 A/dm², preferably in a range between 6 and 8 A/dm².

Also important for the subsequent forming process of the producedgalvanic coating is the chloride content in the electrolyte bath. Thisshould be more than 24 g/l, preferably more than 30 g/l.

Another preferred version of the procedure proposes that the cobaltalloy contain the ingredients of nickel, iron, tin, indium, palladium,gold and/or bismuth that reduce the contact resistance of the subsequentbattery shell manufactured by a forming process. These ingredients canbe used as a simple admixture or in coatings made of more than two ofthese elements.

A raw material especially suited for the production of the cold band issteel with a carbon content of less than 0.20% of a thickness of 1 mmand preferably of 0.1-0.7 mm. According to a first version, the brittlecoating can be deposited directly on the base material made of steel.According to a second version, the brittle coating is deposited on alayer arranged underneath and deposited previously in a galvanicprocess. This layer arranged underneath is preferably anelectrolytically deposited, and subsequently homogenized nickel layerthat demonstrates an excellent corrosion resistance. Afterwards, thematerial is coated with a layer of a high cobalt content with embeddeddecomposition products of organic ingredients. Although during thesubsequent strong forming process by deep-drawing or drawing and ironingthe cobalt layer cracks, such cracks do not reach all the way down tothe steel base material but, rather, they are stopped by the underlyingductile nickel layer so that the corrosion resistance remains intact.

In another version, the layer according to this invention with a highcobalt content is coated with an additional layer that leads to a bettersurface conductivity of the battery shell manufactured from the bandmaterial by deep drawing. Suitable for such coating is, e.g., gold orpalladium. In this version of the cold band too, the hard, brittlecobalt layer including the additional coating of gold or palladium (thatit carries) cracks during a subsequent deep drawing or drawing andironing. In this manner, the advantage of a brittle layer with regard tothe electrical contact resistance of the band material can be combinedwith the advantage of a particularly good conductivity of the surface.

Furthermore, it is possible—before the deposition of the brittle layerwith a high cobalt content—to coat the material with a layer withembedded, electrically conductive particles such as carbon particles. Inthis case, the subsequent cracking of the coating creates fissures thatreach down to the electrically conductive particles in the material, bywhich process the particles are now partially located on the surface ofthe band material. In this variant, the positive properties of a coatingwith imbedded particles such as carbon particles with regard toconductivity are combined with the previously described positiveproperties of the coating with a high cobalt content that will be latercracked.

Another preferred variant of the procedure according to this inventionproposes that the layer of cobalt or a cobalt alloy be applied onto bothsides of the cold band, that both coatings occur in the same electrolytebath but with different current densities so that the current densityduring the coating of the side that will later form the outer side ofthe battery shell is set up differently from the current density duringthe coating of the side that will later form the inner side of thebattery shell. The additional layer on the subsequent outer side of thebattery shell brings substantial advantages during the deep drawing ordrawing and ironing of the battery shell because the danger for theparticles to deposit on the deep-drawing tool is reduced. In this way,during a single treatment step the good drawing properties of a metalsheet with a cobalt coating are combined with good properties withregard to the performance of the final battery achieved due to thebrittle coating on the inner side.

This result can also be achieved by first applying such a coating to atleast the side of the cold band that will later form the outer side ofthe battery shell, that contains nickel grains of a smaller size due to,e.g., the addition of grain refining agents, and then homogenizing thematerial. A suitable grain-refining agent is, e.g., para toluolsulphonamide. In a second procedure step, i.e., after the homogenizingor possibly rerolling or killing, the above described brittle layer ofcobalt or a cobalt alloy is applied on the side of the cold band thatwill later form the inner side of the battery shell. The thus producedmaterial combines the material properties of the outer side advantageousfor deep-drawing and drawing and ironing process with the requiredexcellent contact capability on the inner side of the subsequentlyformed battery shell.

During the procedure according to this invention the organic ingredientsin the electrolyte disintegrate into decomposition products due to thestream flowing in the electrolyte during the galvanization process.These decomposition products can react with other components of thegalvanic bath, especially with metal ions. The thus obtained reactionproducts are deposited on the cold band together with otherdecomposition products and with the cobalt or cobalt alloy, and cause animbrittlement of the coating. In case of organic substances containingsulfur or carbon, these reaction products might be, e.g., cobaltsulfides or cobalt carbides.

Among organic ingredients suitable for addition to the cobalt-containingelectrolyte are brighteners known from the galvanic nickel-platingprocess. Also suitable are preferably brighteners called secondarybrighteners. A typical example of this group is butyne diol. Galvanicdeposits with these ingredients in the cobalt electrolyte bath result ina very hard and, at the same time, brittle coating, which is why thematerial tends to form strong cracks during the subsequent formingprocesses. The arising fissures are characterized by a relativelyhomogeneous structure with a lozenged shape of the individual fissureplates.

During the tests, after the cold band produced according to thisinvention has been subjected to a deep-drawing process, the averagelength of the plate edge turned out to be between 3 and 50 μm. The formand the edge length of the produced lozenge-shaped fissure plate is ofdecisive importance for the performance of the batteries subsequentlymanufactured from the band material.

In this connection, it is of a special advantage that, during theforming process to manufacture a battery shell, the brittle coating ofthe cold band produced by a procedure according to this invention alwayscracks in such a manner that the fissures run not in the longitudinaldirection of the battery shell but rather at an angle of about 45° tothis direction. This is of a particular importance, because during themanufacture of the batteries a cathode mass is pressed into the batteryshell, which has been previously pressed into the so-called “pellets”.These pellets are rings or disks made of a mixture of manganese dioxide,carbon, caustic potash solution and a binding agent. A functionalcontact for the conducting of electrons is endeavored during thepressing of the rings into the battery shell. Since during the formingprocess the band material produced by a procedure according to thisinvention forms fissures at an angle of 45° to the forming direction,during the pressing of the pellets into the battery shell a portion ofthe cathode mass can enter the fissures running at an angle to thepressing direction. This circumstance results in an especially goodcontact between the cathode mass and the inner side of the batteryshell. This advantage of an improved contact is combined with theadvantage of a good storage life of a battery shell coated with cobalton its inner side. As a result, this allows manufacturing batteries thatare characterized not only by an excellent contact of the cathode masswith the inner side of the battery shell due to the cracked surface, butalso by an excellent storage life due to the cobalt coating. Thisapplies accordingly also to a graphite layer arranged on this coatingbefore the filling of the battery shell with the cathode mass, where,however, it is not the direct contact of the cathode mass with thebattery shell but rather the contact of the graphite layer with thebattery shell due to a strong anchoring of the graphite on the innerside of the battery shell.

FIG. 3 shows a table of examples of coated cold bands produced with theparameters of this invention. The organic ingredients to the electrolytebath in FIG. 3 designated with V1 to V4 are as follows:

-   -   V1: butyne diol    -   V2: A mixture of butyne diol and saccharine. In the table of        FIG. 3, in the cell “concentration” the first number relates to        the concentration of butyne diol, and the second number relates        to the concentration of saccharine.    -   V3: Para toluol sulfonamide    -   V4: Saccharin

In addition, FIG. 3 shows, among other things, the pH value,temperature, chloride content, the concentration and the current densityof each particular electrolyte bath used. And finally, the tableindicates each particular behavior of the coated cold band during thesubsequent forming process of deep drawing or drawing and ironing aswell as the average edge length of the lozenge-shaped fissure platesarising during the deep-drawing process. These edge lengths are, e.g.,also illustrated in FIG. 2.

FIG. 1 shows an enlarged fissure pattern of example 27 marked with an“*” in FIG. 3, FIG. 2 shows example 9 marked with “**”.

1. A process for manufacturing battery shells, comprising the steps of:depositing a nickel layer electrolytically on a cold band; coating thecold band with a cobalt-containing layer in a galvanic process, andforming the coated cold band into a battery shell, wherein, organicsubstances are added to the electrolyte that create decompositionproducts, and that these decomposition products and/or reaction productsof these decomposition products with other components of the galvanicbath are deposited together with the cobalt-containing layer on the handmalarial as a brittle layer, and wherein the thus produced brittlecoating laser cracks and forms fissure plates during the forming stop.2. The process of claim 1, wherein the cold band comprises a steel typeof a carbon content of less than 0.2% and of a thickness of up to 1 mm.3. The process of claim 1, wherein the deposited nickel layer is firsthomogenized.
 4. The process of claim 3, wherein a grain refining agentis added to the galvanic bath during the electrolytical depositing ofthe nickel layer contains electrically conductive particles.
 5. Theprocess of claim 1, wherein the nickel layer contains electricallyconductive particles.
 6. The process of claim 5, wherein theelectrically conductive particles are carbon particles.
 7. The processof claim 1, wherein the organic electrolyte ingredients are brighteners.8. The process of claim 7, wherein the brightener comprises butyne diol.9. The process of claim 1, wherein the current density of theelectrolyte bath is less than 10 A/dm².
 10. The process of claim 1,wherein the chloride content in the electrolyte bath is higher than 30g/l.
 11. The process of claim 1, wherein the cobalt coating layer isapplied onto both sides of the cold band, with both coatings occurringin the same electrolyte bath but at different current densities, wherethe current density during the coating of the side that will later formthe outer side of the battery shell is set up differently from thecurrent density during the coating of the side that will later form theinner side of the battery shell.
 12. The process of claim 1, wherein thecobalt-containing coating is applied using an electrolyte comprisingcobalt.
 13. The process of claim 1, wherein the cobalt-containingcoating is applied using an electrolyte comprising a cobalt alloy. 14.The process of claim 13, wherein the cobalt content in the electrolyticcoating is higher than 35 weight percent.
 15. The process of claim 14,wherein the cobalt alloy comprises an admixture of nickel, iron, tin,indium, palladium, gold and/or bismuth.
 16. The process of claim 14,wherein the nickel layer contains electrically conductive particles. 17.The process of claim 15, wherein the cold band comprises a steel type ofa carbon content of less than 0.2% and of a thickness of up to 1 mm. 18.The process of claim 1, wherein the forming step comprises deep drawing.19. The process if claim 1, wherein the forming step comprises drawingand ironing.
 20. The process of claim 4, wherein the grain refiningagent is para toluol sulfonamide.
 21. The process of claim 16, whereinthe electrically conductive particles are carbon particles.
 22. Theprocess of claim 17, wherein the nickel layer contains electricallyconductive particles.
 23. The process of claim 22, wherein theelectrically conductive particles are carbon particles.
 24. The processof claim 3, wherein the nickel layer contains electrically conductiveparticles.
 25. The process of claim 24, wherein the electricallyconductive particles are carbon particles.
 26. The process of claim 7,wherein the brightener further comprises o-benzosulfimide (saccharine).27. The process of claim 9, wherein the current density of theelectrolyte bath is in the range of 6 to 8 Λ/dm².
 28. The process ofclaim 6, wherein the organic electrolyte ingredients are brighteners.29. The process of claim 28, wherein the current density of theelectrolyte bath is less than 10 Λ/dm^(2.)
 30. The process of claim 29,wherein the chloride content in the electrolyte bath is higher than 30g/l.
 31. The process of claim 30, wherein the cobalt coating layer isapplied onto both sides of the cold band, with both coatings occurringin the same electrolyte bath but at different current densities, wherethe current density during the coating of the side that will later formthe outer side of the battery shell is set up differently from thecurrent density during the coating of the side that will later form theinner side of the battery shell.
 32. A battery shell, characterized inthat it is manufactured in a process according to claim 31.